This invention relates to a switching system to power an electric load from a voltage supply. More particularly, for example, the present invention relates to an electronic switch control system for a mine vehicle having controls for selectively connecting the electric motors thereof to a power supply. This invention also relates to an electronic testing system for indicating and locating failures which may occur in an electronic switching system.
For mine vehicle control it is generally known that contactor type systems are used due to their reliability and ease of replacement in a hostile environment. These contactor type systems have been used for controlling both the rotational direction and speed of the motors of mine vehicles. For example, tram motors on the mine vehicle may be connected in series, dividing a 300 volts DC line voltage equally between two tram motors (150 volts DC each). Because the motors are rated for 300 volt line voltage, this reduction in voltage causes the motors to turn at half their normal speed (low tram). A transition to the high speed is accomplished by connecting the motors in a parallel configuration, rather than a series configuration, through use of mechanical interlocks and additional contacts. In parallel the motors are now at 300 volts DC and turn at a higher rate of speed (high tram). Such a contactor controller system is susceptible to contact failure which results in high maintenance cost as well as delays in production during down time. An arc is produced when the contacts are switched, thus producing ozone gas which causes deterioration of wiring, electrical terminations and mechanical components.
One presently available system has reduced the number of switching contacts by utilizing Silicon Controlled Rectifiers (SCRs). This system also has an electronic switching system that varies the effective motor voltage by controlling a series of line voltage pulses. Such a system has some drawbacks. Since the system produces pulses, it is more complex and trouble shooting of the circuits is difficult and time consuming. The training requirements for understanding and trouble shooting the system are greater than for the contactor system. Because of the complexity and greater training requirements, the diagnosing of a failure is a significant maintenance cost. Additional dollar losses occur during the downtime of the mine vehicle since it is not available for production.
As with all SCRs, once triggered into conduction, they remain on until the current flowing through the device is interrupted. Thus, in the presently available SCR systems, which vary the effective motor voltage by controlling the line voltage pulses, during each pulse cycle the SCR must have the current flow interrupted. Circuitry which will provide such interruption of current flow is complex and involves the solution of substantial design problems. Troubleshooting of numerous circuit components is difficult and time consuming. The circuit components are expensive to replace and because of the circuit complexity require extensive training to acquire an understanding of the system sufficient to service it.